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Piston markings!?

er0080

NAXJA Forum User
Location
USA
I'm currently in the process of reringing/honing my 88' 4.0L renix motor and have hit a problem. All my pistons were stamped '50' on the crown which led me to believe that this motor was once bored over to .020" (.50mm) So I purchased the .020" oversize rings and it turns out they are too large (can't make them fit in the cylinders w/out some serious gap filing). The cylinder ridges have all been reamed flush to the uppermost compression ring contact area.

My question to you all: Based on the '50' stamping, are these pistons standard size or are they .020 over? I don't have any means of checking the actual diameter of the bores or the pistons. I suppose I could gap each new ring to within the spec. range, but something tells me these pistons are actually std. size and therefore I should have purchased the std. size rings. any info would be appreciated.



Eric
 
I was once doing this to an '88 XJ and I later found out that the previous owner had a new engine put in. The block and head were out of a '96 engine.
Perhaps you could check the stamp on the side of the block???
 
the motor is original to the car, and as far as I know, the 4.0L comes from the factory only with standard size pistons. The stamped markings on the block are indicative only of over/undersize cam and crank journals.
 
no, none of my mics are large enough for the task. All that im looking for is if anyone knows if the '50' is a standardized crown marking which represents a .50mm oversize (.020"). I do know that in general, numbers that appear stamped on pistons sometimes have no dimensional meaning whatsoever. I was just hoping that someone had experience with the types of markings amc put on their pistons.

Thanks,
Eric
 
er0080 said:
I'm currently in the process of reringing/honing my 88' 4.0L renix motor and have hit a problem. All my pistons were stamped '50' on the crown which led me to believe that this motor was once bored over to .020" (.50mm) So I purchased the .020" oversize rings and it turns out they are too large (can't make them fit in the cylinders w/out some serious gap filing). The cylinder ridges have all been reamed flush to the uppermost compression ring contact area.

My question to you all: Based on the '50' stamping, are these pistons standard size or are they .020 over?

They're standard size. The stock 4.0 pistons have a "50" stamped on the crown and that has nothing to do with them being oversize. You'll need to get rings for standard bore pistons.
 
dyno,

you're right they are the original pistons. I borrowed my bud's bore gauge and 4 inch mic today and actually measured the dimensions myself.

The guy I bought it from (original owner) said he babied it. The engine has the original amc stamped main and rod bearings, as well as amc stamped pistons. Honestly the main bearings didn't need to be changed. Plastigaged them at .0015" oil clearance. The rod bearings had only slight brassing on cylinders 1 and 6. I measured the max cylinder taper to be ~.003" and out of round at ~.0015" (both in cylinder #1) . ring gaps were all over .040" measured at the bottom of ring travel. There was no significant scoring or other damage observed to the pistons/cylinder walls. Not bad considering this is an 88XJ with just over 290k original miles on it!

thanks for the replies
Eric
 
The guy I bought it from (original owner) said he babied it. The engine has the original amc stamped main and rod bearings, as well as amc stamped pistons. Honestly the main bearings didn't need to be changed. Plastigaged them at .0015" oil clearance. The rod bearings had only slight brassing on cylinders 1 and 6. I measured the max cylinder taper to be ~.003" and out of round at ~.0015" (both in cylinder #1) . ring gaps were all over .040" measured at the bottom of ring travel. There was no significant scoring or other damage observed to the pistons/cylinder walls. Not bad considering this is an 88XJ with just over 290k original miles on it!
AMAZING! So why are you re-ringing it? Maybe the rings have lost their tension over the years and it's using oil? The Jeep 4.0 has to be one of the best engines ever made.

50 mm works out to be about 2 inches. Could that be the piston skirt measurement, or the piston depth? We have a mystery here......
 
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